Introduction to Maxfoam process for manufacture of Flexible Polyurethane Foam

The Maxfoam process is the most widely used flexible polyurethane slabstock foam process in the world.

INTRODUCTION

This introduction is designed to give you technical information about the running of the Maxfoam process. It is not intended as an instruction manual for your machine.
This Manual does give information about the main features of the Machine, as they affect the efficient running of the Maxfoam Process. It also gives information on setting up the Machine, chemical formulations and running conditions. There is also a comprehensive section on common faults which you may encounter from time to time, and how they may be overcome.

Safety is an important factor in running an efficient foaming operation.


Basic features of the Maxfoam machine

The basic mechanical features of the Maxfoam machine are shown in Figure 1.01 below.




All chemical streams are metered by accurate pumps and directed to the mixing head port-ring or to the polyol manifold through three-way diverter valves. This arrangement enables metering pumps to run at normal output conditions in a recirculation mode. During foam production, the chemicals are mixed in the mixing head which consists of a variable speed multi-pin stirrer closely fitted inside a cylindrical barrel; the speed can be varied either by a system of changeable pulleys or by an infinitely variable speed controller.

Two fundamental mechanical items are the trough and the fallplate. The chemical mix is fed from the mixing head through two flexible hoses into the bottom of a deep trough. The foam mix is allowed to start reacting and partially expand in the trough before spilling out onto the fallplate section.

The fallplate consists of a number of hinged sections. The vertical position of the trough and each section of the fallplate is controlled by motorised screw-jacks. This arrangement enables the profile shape of the fallplate to be changed to suit different foam formulations.

Onto the fallplate, to contain the foam from the trough, is fed the bottom paper - a continuous sheet of paper from horizontal unwind rolls at the back of the machine. Side papers are also fed from vertical unwind rolls at the start of the sidewalls. The feed tension of the unwind rolls is controlled by adjustable brakes.


From the lower end of the fallplate, the bottom paper runs onto the main conveyor, which is of metal slat construction. The speed of the main conveyor is variable and a critical control for the foaming process.

At the end of the main conveyor, where the side papers are removed from the foam block, are two vertical capstan rolls which are driven at the same surface speed as the main conveyor. These ensure that the side papers travel at the same speed as the foam block. The side papers are wound onto driven rewind rolls. The bottom paper is also removed from the foam block and wound onto a driven rewind roll below the level of the conveyor.

The block cut-off knife or saw is situated after the paper take-off positions. As it cuts, it moves synchronously with the main conveyor so as to ensure an accurate vertical cut.

After the blocks have been cut, they are transported to the cure hall where they will stand for about 12 to 18 hours (depending on the grade of foam) to allow completion of the chemical reactions and cooling of the foam before further processing.


Basic principles of the Maxfoam process

The main difference between Maxfoam and a conventional inclined conveyor slabstock process is the trough. The purpose of the trough is to cause a delay between mixing the chemicals and depositing them on the conveyor. This delay, which is typically about 20 seconds, allows some pre-foaming to occur in the trough, so that a froth instead of a liquid is laid down on the conveyor.


The advantage of this configuration is that the transporting surface where the foam expansion occurs (the Maxfoam Fallplate) can now be much steeper than the conveyor of a conventional machine. Whilst the angle of inclination of a conventional foam conveyor is normally less than 4o, the average inclination of the fallplate is greater than 10o.
The steeper angle is possible because the froth coming out of the trough has a much greater viscosity, and is of lower density than the liquid mix on a conventional machine. These two factors reduce the possibility of under-run, the running of heavier liquid material underneath older, lower density foam.

This steeper angle allows a shorter machine to be used, at lower outputs and conveyor speed - for the same height of block. Density distribution through the block is also improved.

A further advantage of Maxfoam is the ability to control the shape of the top of the block by adjusting trough height and full-rise position.

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